Epoch (geology)
The geological time scale is used by geologists and other scientists to describe the timing and relationships between events that have occurred during the history of the Earth. The table of geologic periods presented here is in accordance with the dates and nomenclature proposed by the International Commission on Stratigraphy, and uses the standard color codes of the United States Geological Survey. Evidence from radiometric dating indicates that the Earth is about 4,570 million years old (expressed with m.y.a., i.e. million years ago, or "Ma" as in "it dates from 4570 Ma"). The geological or deep time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the time scale are usually delimited by major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the extinction event that marked the demise of the dinosaurs and of many marine species. Older periods which predate the reliable fossil record are defined by absolute age. Graphical timelines The second and third timelines are each subsections of their preceding timeline as indicated by asterisks. The Holocene (the latest epoch) and the present (from 1950 to now) are too small to be shown clearly on this timeline. Terminology The largest defined unit of time is the Eon. Eons are divided into Eras, which are in turn divided into Periods, Epochs and Stages. At the same time paleontologists define a system of faunal stages, of varying lengths, based on changes in the observed fossil assemblages. In many cases, such faunal stages have been adopted in building the geological nomenclature, though in general there are far more recognized faunal stages than defined geological time units. Geologists tend to talk in terms of Upper/Late, Lower/Early and Middle parts of periods and other units , such as "Upper Jurassic", and "Middle Cambrian". Upper, Middle, and Lower are terms applied to the rocks themselves, as in "Upper Jurassic sandstone," while Late, Middle, and Early are applied to time, as in "Early Jurassic deposition" or "fossils of Early Jurassic age." The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic." Because geologic units occurring at the same time but from different parts of the world can often look different and contain different fossils, there are many examples where the same period was historically given different names in different locales. For example, in North America the Lower Cambrian is referred to as the Waucoban series that is then subdivided into zones based on trilobites. The same timespan is split into Tommotian, Atdabanian and Botomian stages in East Asia and Siberia. It is a key aspect of the work of the International Commission on Stratigraphy to reconcile this conflicting terminology and define universal horizons that can be used around the world. History of the time scale The principles underlying geologic (geological) time scales were laid down by Nicholas Steno in the late 17th century. Steno argued that rock layers (or strata) are laid down in succession, and that each represents a "slice" of time. He also formulated the principle of superposition, which states that any given stratum is probably older than those above it and younger than those below it. While Steno's principles were simple, applying them to real rocks proved complex. Over the course of the 18th century geologists came to realize that: 1) Sequences of strata were often eroded, distorted, tilted, or even inverted after deposition; 2) Strata laid down at the same time in different areas could have entirely different appearances; 3) The strata of any given area represented only part of the Earth's long history. The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth took place in the late 18th century. The most influential of those early attempts (championed by Abraham Werner, among others) divided the rocks of the Earth's crust into four types: Primary, Secondary, Tertiary, and Quaternary. Each type of rock, according to the theory, formed during a specific period in Earth history. It was thus possible to speak of a "Tertiary Period" as well as of "Tertiary Rocks." Indeed, "Tertiary" and "Quaternary" remained in use as names of geological periods well into the 20th century. The identification of strata by the fossils they contained, pioneered by William Smith, Georges Cuvier, Jean d'Omalius d'Halloy and Alexandre Brogniart in the early 19th century, enabled geologists to divide Earth history more precisely. It also enabled them to correlate strata across national (or even continental) boundaries. If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time. Detailed studies of the strata and fossils of Europe produced, between 1820 and 1850, the sequence of geological periods still used today. British geologists dominated the process, and the names of the periods reflect that dominance. The "Cambrian," "Ordovician," and "Silurian" periods were named after ancient British tribes (and defined using stratigraphic sequences from Wales). The "Devonian" was named for the English county of Devon, and the name "Carboniferous" was simply an adaptation of "the Coal Measures," the old British geologists' term for the same set of strata. The "Permian" was named after Perm, Russia, because it was defined using strata in that region by a British geologist Roderick Murchison. However, some periods were defined by geologists from other countries. The "Triassic" was named in 1834 by a German geologist Friedrich Von Alberti from the three distinct layers (Latin trias meaning triad) —red beds, capped by chalk, followed by black shales— that are found throughout Germany and Northwest Europe, called the 'Trias'. The "Jurassic" was named by a French geologist Alexandre Brogniart for the extensive marine limestone exposures of the Jura Mountains. The "Cretaceous" (from Latin creta meaning 'chalk') as a separate period was first defined by a Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris basin and named for the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates). British geologists were also responsible for the grouping of periods into Eras and the subdivision of the Tertiary and Quaternary periods into epochs. When William Smith and Sir Charles Lyell first recognized that rock strata represented successive time periods, time scales could be estimated only very imprecisely since various kinds of rates of change used in estimation were highly variable. While creationists had been proposing dates of around six or seven thousand years for the age of the Earth based on their Christian heritage, early geologists were suggesting millions of years for geologic periods with some even suggesting a virtually infinite age for the Earth. Geologists and paleontologists constructed the geologic table based on the relative positions of different strata and fossils, and estimated the time scales based on studying rates of various kinds of weathering, erosion, sedimentation, and lithification. Until the discovery of radioactivity in 1896 and the development of its geological applications through radiometric dating during the first half of the 20th century (pioneered by such geologists as Arthur Holmes) which allowed for more precise absolute dating of rocks, the ages of various rock strata and the age of the Earth were the subject of considerable debate. In 1977, the Global Commission on Stratigraphy (now the International Commission) started an effort to define global references (Global Boundary Stratotype Sections and Points) for geologic periods and faunal stages. The commission's most recent work is described in the 2004 geologic time scale of Gradstein et al. (ISBN 0-521-78673-8), and is used as the basis of this page. More information on the development of the geologic time scale can be found in the articles history of geology and history of paleontology. Table of geologic time (This table is not to any scale.) References and footnotes * GeoWhen Database * International Commission on Stratigraphy Time Scale * CHRONOS * CHRONOS Geologic Time references * National Museum of Natural History - Geologic Time * BBC Interactive Time Line See also * Age of the Earth * Timeline of evolution * Cosmological timeline * Lunar geologic timescale * Martian geologic timescale * Anthropocene * Logarithmic timeline *Timetable of the Precambrian *Time line of the geologic history of the United States External links *NASA: Geologic Time *GSA: Geologic Time Scale *British Geological Survey: Geological Timechart Category:Earth sciences Category:Geology Category:Geochronology Category:Geologic time scale Category:Geology timelines